Most Redditors are college-educated young men living on the west coast with a moderate income. They are hip, smart, and like to talk about whatever is trending. (If you are a sci-fi writer, Redditors are your audience.)
Aside from being the best place to get the latest news, Reddit is a great resource if you know how to navigate it.
At first it may seem like a manic hodge-podge of random questions, comments, and news items. There is that, but there is also a world of mind-expanding specialized knowledge. (The most popular reddits, however, have nothing to do with any consistent topic.)
There is a wide range of subreddits for those who are looking for specific topics and areas of interest, some of which are well-trafficked, and some that are voices crying in the wilderness - for example, those having to do with writing. But, as it happens, sometimes a small, intimate group can be more productive than 8 million subscribers who simply want a laugh. (Funny is reddit's most popular subreddit.)
The best way for writers to use reddit is not as a marketing tool. It's true there are subreddits that will allow you to post your freebies, but the best use you can make of Reddit is as a source of information and/or critique. If you are looking for publishing or marketing advice, for example, you can ask a question and get useful - and accurate - answers from professionals and from people who are willing to share their personal experience.
There are also a number of active writing critique groups on Reddit. The critiques I've read have been right on the mark, and written by people who clearly understand the craft of writing. You may even find yourself willing to offer some suggestions of your own.
So, check out the subreddit list below. (Please note that the reader numbers are low on this list as Reddit has not updated this hub recently. All of the listed subreddits are still up and running.)
From the website:
/r/WritingHub is an index of writing subreddits designed to help writers find communities relevant to their interests. Moderators are welcome to announce contests, AMAs (Ask Me Anything), or any other events going on in their communities, and new communities are always welcome to introduce themselves. (Some of these subreddits are more active than others, so pick the ones that have recent posts.)
Not on this list, but deserving of mention is /r/books. This is where books get discovered.